Portugal is the largest cork producer and processor worldwide. Today, cork stoppers account for the highest sales volume in the cork industry. The economic valorisation of cork stoppers will enable a sustainable maintenance and exploitation of the cork oak plantation, also contributing to biodiversity preservation, CO2 fixation, and creation of local employment.
However, although cork stoppers contain unique properties, a percentage of them give the so-called “cork taste” to the wine, a very unpleasant (smell and taste) organoleptic defect, which reduces the wine quality and its acceptance by the consumers, thus causing serious losses to the relevant wine brand and, consequently, to the cork sector.
Several organohalogenated compounds, in particular 2,4,6-trichloroanisole (TCA), have been described as responsible for giving the wine some unpleasant organoleptic properties. Cork stoppers have been pointed out as being at the root of that contamination.
TCA has been identified as the main responsible for the problem of “cork taste” in the wine for the first time by Tanner et al. in 1982, and although a relation has also been established between other aromatic compounds and this defect (Simpson, 2004; Pena-Neira 2000; Chatonnet, 2004), TCA is generally considered as the best indicator of problems in the wine (Herv é, 2000). According to the European project Quercus, the presence of TCA was detected in at least 80% of the wines with “cork taste”. In addition to this, TCA is a compound whose sensorial detection limit is set in very low concentrations, in the range of nanograms per liter of wine (Amon, 1989; Tanner, 1982).
Other organochlorine compounds are also highlighted, such as 2,4,6-trichloroanisole (2,4,6-TCA), 2,3,4,6-tetrachloroanisole (TeCA), pentachloroanisole (PCA), 2,4-dichloroanisole (2,4-DCA), 2,6-dichloroanisole (2,6-DCA), 2,4,6-trichlorophenol (2,4,6-TCP), 2,3,4,6-tetrachlorophenol (2,3,4,6-TCP), 2,4,6-tribromoanisole (2,4,6-TBA) and pentachlorophenol (PCP); from other chemical groups, there are compounds such as 2-methoxy-3,5-dimethylpyrazine (MDMP), guaiacole, geosmine, 2-methyl-isoborneol and 1-octen-3-ol. Significantly, 80 to 85% of the so-called “cork taste” is attributed to the presence of 2,4,6-trichloroanisole (2,4,6-TCA). In this context, eliminating those contaminants in general, and 2,4,6-TCA in particular, from the cork stoppers is extremely important for the cork industry.
The presence of TCA in the cork stopper has been the object of several studies (Silva Pereira, 2000), along with the development of processes intended for treating and mitigating the damage (Gil, 2006). The direct precursors of chloroanisoles are chlorophenols which are converted into chloroanisoles by means of a methylation reaction carried out by some microorganisms, especially fungi under specific conditions of temperature and pressure (Insa, 2006). Therefore, chlorophenols are a potential source of TCA.
The true origin of the organochloride compounds—TCA precursors—found in the cork not being clarified yet, efforts have been focused in the development of methods aimed at removing the said compounds from cork stoppers.
The use of extraction processes is herein highlighted, which are basically focused in procedures intended for the removal of chlorinated compounds from cork by means of extraction/washing with solvents, evaporation or degradation, upon the manufacturing.
The patent WO 2004014436 relates to the extraction of compounds dragged in water vapour from cork granulates, with extraction efficiencies of up to 90% or, as mentioned in the patent WO 03041927, the extraction by ethanol/water in the vapour phase, in natural cork stoppers requiring special care in order to preserve its mechanical, physical and functional properties, with an extraction efficiency of up to 80%.
The patent FR19990012003 discloses an invention in which is perfectly clear that the cork is subject to supercritical extraction, always in the form of board or granulate and not in the form of cork stopper. Actually, in the whole document there is no reference made to the process being directly applied to the natural cork stopper; a publication by Eduard Lack, in 2006, at the 3rd International Meeting of Chemical Engineering and High Pressure, on behalf of NATEX, the Austrian company which leads the project and the construction of the industrial facilities where the patented supercritical extraction process is carried out, states that during the depressurization step the cork stoppers do not return to their original shape. So, the author concludes that it is impossible to apply this kind of treatment to the natural cork stopper. Furthermore, the author specifies that the treatment with supercritical fluids shall be directly applied to the cork granulate and that cork stoppers must be produced a posteriori.
The extraction with n-pentane in soxhlet is also known, which although effective (being used in laboratory for the analysis of 2,4,6-TCA by GC) involves a very expensive industrial technology (implementation and maintenance), also implying risks related to the solvent's handling and to a possible contamination of the cork.
Microwaves are also used, so that the temperature is increased and the evaporation of the contaminants is made easier. However, 2,4,6-TCA, just like most of trichloroanisoles, has a low volatility (boiling point=240° C.) and is strongly adsorbed to the cork's cellulose, lignin and suberin macromolecules, which makes it hardly dry-desorbed by evaporation. On the other hand, as cork is an excellent thermal insulator, high external temperatures must be used in order to attain the desired internal temperature (which is lower), except in the case where microwaves are used, which can cause an outer deterioration and refraction, with the release of inner compounds becoming more difficult.
In a distinct approach, the irradiation of the cork, namely with gamma radiation (Co60, 15 KGy) (PT103006), ionizing radiation or electron beam, is used for reducing the microbiological contamination (sterilization). These techniques also cause (depending on the dose) the contaminants to degrade, generally transforming them in molecular waste with no smell. However, although the sterilization leads to a decrease in the formation of 2,4,6-TCA and the technique is also susceptible of providing degradation of contaminants, the latter is only partial and the toxicity of the degradation products is unknown. But, above all, those techniques are expensive and, in practical terms, they are impossible to implement in the industry, either due to imposed costs and technical complexity, or to the costs associated to an eventual service being provided.
The use of ozone (Vlachos, 2007), ultrasounds (Penn, C, 2004) and photodegradation (Vlachos, 2008) have been also described.
The use of enzymes for the phenol inactivation has also been tested, namely phenol-oxidase, such as suberaseT (Novozyme). This enzyme promotes the phenol polymerisation, but the strategy has shown to be of very low efficiency as regards the elimination of the “cork taste”.
Another proposed strategy is based on the use of fungi which can inhibit the development of other populations that have been mentioned as being susceptible of conferring unpleasant properties to the cork or whose metabolism can degrade the compounds involved in this contamination process.
In the patent WO2008042181 another technique is mentioned in which the coating of the cork stoppers has a thin film of silicone aimed at ‘encapsulating’ the contaminants thus avoiding their migration. Although simple and not much expensive, the technique proved to be ineffective since most of the cork stoppers undergo a final treatment with paraffins and silicones for purposes of adorning, as well as easing the operation of the cork machine. On the other hand, there is a risk of the contaminants passing from the polymer to the liquid medium when in contact, or even a problem of durability of the coating layer, the treatment of the surface with silicone.
In contrast, over the last 20 years, scientific evidence has been gathered which suggests that the organohalogenated compounds found in nature are extensively produced by microorganisms, namely soil bacteria, yeasts, moulds and filamentous fungi. Most of the organohalogenated compounds produced are environmentally harmless, due to their slow degradation, but some of them have biological activity, thus inhibiting for example the growth of competitive microorganisms. And this is the reason why, in another approach, the development of formulations was sought in such a way as to include the inhibitor(s) of the enzymes that are directly or indirectly involved in the formation of the precursor 2,4,6-TCP.
Furthermore, although some of the aforementioned inventions do contribute to reduce the content of contaminants in the cork stopper, none of the disclosed methods shows to fully prevent their transmission to the wine.
A different perspective is based upon non-invasive and non-destructive control methods, an example of which is the patent WO2011078714. In effect, the heterogeneous character of cork, characteristic of natural products in whose complexity participate inter alia physiological, biological and climatic factors, causes the statistical control, although being relevant to the development and support of a controlled process, to always make available limited information in what concerns the quality of the product. The market requirements as regards the bottle to bottle—stopper to stopper—warranty cannot be attained by the statistical control.
However, this kind of approach is not duly explored in what concerns sensorial deviations in cork stoppers. The patent application WO2005047853 was identified, which relates to the same issue but proposes a different solution from the one disclosed in this invention, mentioning the use of ‘nose chips’ for detection of an analyte in cork stoppers, the said patent not having as object the rhythm of the performed analysis, which is crucial in order to attain the industrial objectives.
The inspection of cork stoppers according to their sensorial quality—the presence of sensorially offensive compounds—is also described in the patent US2008245132. However, this patent only describes a possible classification process, without any specific description of an analytic solution and synchronization of the required events in that scope. Therefore, this patent contains elements which are much more evolved as regards the operation of a 100% and non-destructive inspection in batches of cork stoppers.
The patent WO2004004995 was also identified, in which a decontamination process of the cork is mentioned (TCA and chlorophenols), however not having as its object the inspection, categorization and separation of the unaccepted cork stoppers.
In the patent US2009180122, a method is used for the analysis of a sample by means of fast-scan continuous wave terahertz spectroscopy which is intended for the non-destructive assessment of materials, such as animal skin and natural cork, as well as for the detection of explosives, concealed weapons and narcotic drugs.
Finally, the patent WO2004076607 was also identified, which relates to a process for preventing the cork stoppers' flavours from being transferred to the wine, by the use of a membrane.